In some engines, an electronically controlled throttle is used for improved performance. In particular, the electronic throttle is used to control airflow to a desired value determined from operating conditions and an operator command. In this way, the vehicle can achieve improved drive feel and improved fuel economy.
In this system, the required airflow is used to determine an initial setting of the throttle. Also, a difference between required airflow and actual measured airflow is used to adjust the initial setting of the throttle. Thus, the throttle is used to control airflow and thereby engine torque. Such a system is described in U.S. Pat. No. 5,019,989.
It is also known to have a variable camshaft timing mechanism to adjust engine breathing and residual burnt gas fraction. In this system, camshaft timing is generally determined as a function engine speed and engine load. Position the camshaft as a function of speed and load is used to optimize steady state performance giving minimized emissions and fuel consumption. Such a system is disclosed in U.S. Pat. No. 4,856,465.
The inventors herein have recognized a disadvantage with the above approaches. In particular, a disadvantage with using throttle position is that the throttle cannot quickly change engine torque since the throttle controls flow entering an intake manifold.
Controlling flow entering the manifold cannot rapidly control cylinder charge due to manifold volume. For example, if the throttle is instantly closed, cylinder air charge does not instantly decrease to zero. The engine must pump down the air stored in the manifold, which takes a certain number of revolutions. Therefore, the cylinder air charge gradually decreases toward zero.
Also, the inventors herein have recognized that prior art approaches for controlling variable camshaft timing systems sacrifice dynamic performance. In other words, since the variable cam timing is scheduled versus engine speed and load to provide optimum steady state performance, transient operation is sub-optimal and many opportunities go overlooked. Further, these methods for providing variable cam timing steady state setpoint control are done without regard to manifold dynamics and cause degraded performance.